This invention relates to a record/reproduce circuit for a VTR (video tape recorder) having special reproducing functions such as field still, fine slow motion and so on.
The helical scan type VTR has normally two magnetic heads of different azimuth angles which alternately scan first and second fields. Therefore, in the normal mode in which the magnetic tape travels at a normal speed for reproduction, a video picture can properly be reproduced from the tape. In a special reproduction mode such as the still reproduction mode or the fine slow motion reproduction mode in which the still reproduction and the play condition are repeated, the still reproduction is performed for each frame unit, and thus the same first and second fields of each frame are reproduced alternately. In this connection, since the first and second fields occur at intervals of 1/60 second, the first and second-field images of a high-speed motion picture are alternately reproduced, giving rise to a blur of image so that the still picture quality is quite poor. Moreover, in the still reproduction mode, although the magnetic heads scan two adjacent tracks on the magnetic tape from one to the other, since the adjacent tracks are formed by the magnetic heads with different azimuth angles, part of the tracks cannot be reproduced substantially even although the tracking volume control is turned. This gives rise to noise on the screen, reducing the noise drive margin.
Thus, recently, there has been developed a VTR having one or two additional heads provided so that two magnetic heads with the same azimuth angle can scan the same field track, as disclosed in the Japanese magazine, "Television technology", 1980, December, pages 45-50. According to this prior technique, in the still reproduction mode, the same field track is reproduced by two magnetic heads with the same azimuth angle, or field still reproduction is performed.
Thus, in the fine slow motion mode, since still and play reproductions are repeated alternately, it is necessary to switch the magnetic heads upon reproduction. In the above technique, in order that a switching noise is not caused by DC level difference at the head switching time point, preamplifiers the number of which is equal to that of heads used are provided to amplify the head outputs sufficiently and at the succeeding stage thereof the switching of heads is performed.
A conventional example of VTR will hereinafter be described with reference to FIG. 1. This example employs normal reproducing heads 1 and 2, and a special reproducing head 3. The heads 1 and 3 have the same azimuth angle, and the head 2 has a different azimuth angle from that of the heads 1 and 3. The heads 1, 2 and 3 are connected through rotary transformers 4, 5 and 6, resonating capacitors 7, 8 and 9 for adjusting the resonant frequencies, and damping resistors 10, 11 and 12 for adjusting the quality factors at the resonant points, to preamplifiers 13, 14 and 15, respectively. The resonating capacitors 7 to 9, and damping resistors 10 to 12 are provided to perform the peaking adjustment for the heads 1, 2 and 3, respectively. The reproduced outputs, b and c from the heads 2 and 3 are first applied to a first switching circuit 16 for switching the normal and special reproduction modes. The output of the switching circuit 16 and the output, a from the head 1 are applied to a second switching circuit 17, which selects the head having greater contact with the tape at each field. FIG. 2 shows the positions at which the heads 1, 2 and 3 are mounted on a rotary cylinder 18. The special reproducing head 3 is located at A, or B before or after the normal reproducing head 2. The distance between the magnetic gaps of the heads 2 and 3 on the circumference of the cylinder 18 is usually within 3H (1H is the distance corresponding to one horizontal scanning period on the circumference of the cylinder) so that horizontal skewing is not caused.
The drawbacks of the conventional example as shown in FIG. 1 include the requirement of: (1) the peaking adjustment for each of the three heads; and (2) three preamplifiers, which requirements increase cost of the VTR.